1. Field of the Invention
The present invention relates to a proton exchange membrane fuel cell stack.
2. Description of the Related Art
Proton exchange membrane fuel cells (PEMFCs) include three constituents forming a basic unit of membrane and electrode assembly (MEA): an anode electrode for oxidation reaction, a cathode electrode for reduction reaction, and an electrolyte for ion transportation.
The anode electrode and cathode electrode include a gas diffusion layer, which promotes the supply of hydrogen and oxygen, and a catalyst layer, which induces chemical reactions and transfers the generated ions. Platinum (Pt)-impregnted carbon with a high reaction activity even at a low temperature is used for the catalyst layer. A porous carbon paper coated with a microporous layer enabling uniform fuel diffusion is used for the gas diffusion layer.
The electrolyte is formed as a fluorinated organic polymer membrane with a high ionic conductivity. A composite membrane with a porous polytetrafluoro ethylene (PTFE) layer and a non-fluorinated organic polymer membrane are also available as the electrolyte.
One of the features of fuel cells such as PEMFC is a considerably high current density per area, but a very low theoretical potential for each MEA. The potential required to induce a sufficient current for use is less than 1V for each MEA. Accordingly, a few to tens of MEA's should be connected in series to obtain a desired potential level for electronic device and electric vehicle applications.
In the manufacture of a stack of multiple cell units, graphite has been widely used due to its excellent electrical conductivity. Also, graphite has good machining property, so it provides a convenience in processing a fine, complex fuel flow field on graphite.
Although graphite have the above-listed advantages, use of graphite in the fabrication of a cell stack is limited by the following problems. In terms of a current need to reduce cell volume and weight per power yield (kW), using graphite as a major constituent of a cell stack is undesirable. Disadvantageously, graphite has a small hardness, so it is easily broken. Therefore, when stacking tens of cell units with the application of a very high pressure, special cautions are needed to prevent individual cell units from being broken. When fuel supply to a certain cell unit of the stack of tens of cell units is not smooth or when the catalyst layer of one cell unit shows a reduced reactivity due to a defect, it is impossible to repair or replace the defective cell unit in a conventional cell stack due to its structural limitations. Although the defective cell unit can be removed from the stack, significant degradation of performance is caused by disassembling and assembling processes.
A conventional stack of cell units has a structural limitation, so it is difficult to stack individual cell units. It is crucial to tightly seal the stack to prevent leakage of fuel gas through the gap between a bipolar plate and MEA. There are complex requirements for a gasket for use in sealing; for example, the gasket should have an appropriate thickness and flexibility and should be durable against chemical reaction without degradation.